Ion carrier membrane, and ion sensor having same

ABSTRACT

The surface of an electrically conductive substrate (1) is provided with a redox layer (5) by an electrolytic polymerization process, and the surface of the resulting member is further provided with an ion carrier membrane (6) which includes polyvinyl chloride, a plasticizer and an ion carrier, thereby forming an ion sensor. Used as the plasticizer is one selected from among phthalate plasticizers, maleate plasticizers, adipate plasticizers, carboxylate plasticizers and polycaprolactone plasticizers, or one selected from among plasticizers comprising modified ethylene - vinyl acetate copolymer. The ion sensor obtained has low solubility in the plasticizer of the redox layer and therefore is highly stable. Since the ion sensor has little toxicity, it can be used in vivo. A preferred plasticizer is benzopohenon-tetracarboxylic under cyclic acid.

TECHNICAL FIELD

This invention relates to an ion carrier membrane. More particularly,the invention relates to an ion carrier membrane which exhibits ionselectivity in an ion sensor, ion-selective FET sensor (ISFET) and thelike, and to an ion sensor equipped with such an ion carrier membrane.

BACKGROUND ART

With regard to the action of a plasticizer in the membrane compositionof a polymeric ion carrier membrane in a liquid membrane-type electrodeof an ion sensor or the like in which the polymeric membrane issubstituted for a glass membrane, W. Simon et al have found that thedielectric constant of the membrane solution (plasticizer) (awater-immiscible liquid of low vapor pressure, compatible with PVC, nofunctional group of which can undergo protonation reaction) enhances theselectivity of a monovalent cation of the same size ["Ion EnzymeElectrodes in Biology & Medicine", W. Simon, Int. Workshop, pp. 22-37(1976)]. As a result, o-NPOE [o-nitrophenyl-n-octylether 24 (20° C.)] isused as the plasticizer, and this is put to use in an Na⁺ sensor and K⁺sensor. It was subsequently reported that DOS (dioctyl sebacate), whichhas a high lipophilic/hydrophilic distribution coefficient, is bestsuited for an ion-selective electrode [Anal. Chemistry 52 (4) 692-700(1980)]. However, this concept does not hold true in a case where theion carrier membrane is of a solid-state type. In such case, thestability between the ion carrier membrane and an electrolytic polymericmembrane takes on great importance.

In the prior art, it is known to incorporate an ion carrier substance inthe ion carrier membrane used in an ion-selective electrode, in which apolyvinyl chloride resin serves as a matrix and a plasticizer serves asthe solvent. Examples of the plasticizer which can be used are phthalateesters, maleates, sebacates and adipates, though flow-out of theseplasticizers and the toxicity of the effluent pose problems. Inparticular, it is known that the amount of plasticizer outflow increasesin blood or body fluids. For this reason, a plasticizer which exhibitslittle outflow is desirable as the membrane for an ion sensor of thetype used to subject blood or body fluids to measurement.

In Japanese Patent Application Laid-Open Nos. 61-155949 and 61-194343,etc., the inventors have previously reported on an ion sensor obtainedby depositing a redox layer on an electrically conductive substrate, andcoating the result with an ion carrier membrane. From the standpoint ofdurability, it is desired that the plasticizer in these ion carriermembranes have little solubility with respect to the redox layercomposition.

DISCLOSURE OF THE INVENTION

The present invention provides an ion carrier membrane and an ion sensorhaving the same, which ion carrier membrane improves the stabilitybetween itself and an electrolytic polymeric membrane through use of aplasticizer which does not readily dissolve in a solution and whichexhibits ionic mobility in the ion carrier membrane.

Further, the present invention provides an ion carrier membrane and anion sensor having the same, which ion carrier membrane reduces theoutflow of materials to blood or the like and raises the durability ofthe ion sensor through use of a plasticizer which does not readilydissolve in a solution under measurement and which exhibits littlemutual solubility with respect to an electrolytic polymeric membrane(redox layer).

As means for solving the foregoing problems, the ion carrier membrane ofthe invention is an ion carrier membrane sensitive to a prescribed ionand comprising polyvinyl chloride plasticized by a plasticizer, theplasticizer being selected from at least one of maleate plasticizers,and carboxylate plasticizers.

Further, the ion carrier membrane of the invention is an ion carriermembrane sensitive to a prescribed ion and comprising polyvinyl chlorideplasticized by a plasticizer, the plasticizer being selected frompolycaprolactone plasticizers and plasticizers comprising modifiedethylene--vinyl acetate copolymer.

Further, an ion sensor according to the invention comprises anelectrically conductive substrate, a redox layer exhibiting a redoxfunction coating the electrically conductive substrate, and an ioncarrier membrane coating the redox layer, wherein the ion carriermembrane is sensitive to a prescribed ion and comprises polyvinylchloride plasticized by a plasticizer, the plasticizer being selectedfrom at least one of phthalate plasticizers, maleate plasticizers,adipate plasticizers, carboxylate plasticizers, polycaprolactoneplasticizers and plasticizers comprising modified ethylene--vinylacetate copolymer.

Further, an ion sensor according to the invention comprises a gateinsulation layer of a FET, an electrically conductive layer coating thegate insulation layer, a redox layer exhibiting a redox function coatingthe electrically conductive substrate, and an ion carrier membranecoating the redox layer, wherein the ion carrier membrane is sensitiveto a prescribed ion and comprises polyvinyl chloride plasticized by aplasticizer, the plasticizer being selected from at least one ofphthalate plasticizers, maleate plasticizers, adipate plasticizers,carboxylate plasticizers, polycaprolactone plasticizers and plasticizerscomprising modified ethylene--vinyl acetate copolymer.

By virtue of this construction of the invention, the ion carriermembrane enhances the mutual stability between itself and anelectrolytic polymeric layer through use of the aforementionedplasticizers, which do not readily dissolve in solution and whichexhibit ionic mobility in the ion carrier membrane.

Further, the ion carrier membane is prevented from eluting into anelectrolytic polymeric layer (redox layer) and the stability thereofwith respect to the electrolytic polymerization layer is raised by usinga plasticizer selected from polycaprolactone plasticizers andplasticizers comprising modified ethylene--vinyl acetate copolymer,which plasticizers are polymeric, do not readily dissolve in solution,plasticize polyvinyl chloride in the ion carrier membrane and arecompatible with the ion carrier material.

Preferred embodiments of the invention are as follows:

(1) The weight ratio of plasticizer to polyvinyl chloride is100-500:100.

(2) A phthalate having a carbon number of from 4 to 14, particularlydi(2-ethylhexyl) phthalate (DOP), is used as the phthalate plasticizer.

(3) Dioctyl maleate (DOM) is used as the maleate plasticizer.

(4) Dioctyl adipate (DOA) is used as the adipate plasticizer.

(5) Adipate polyester is used as the adipate plasticizer.

(6) Benzophenontetracarboxylic undecylic acid (BTCU) is used as thecarboxylic plasticizer.

In accordance with the invention, there can be provided an ion carriermembrane, and an ion sensor having the same, in which the ion carriermembrane is endowed with improved stability with respect to anelectrolytic polymeric layer by using a plasticizer which does notreadily dissolve in solution, and which exhibits ionic mobility in theion carrier membrane.

The following results are obtained:

(1) It has been found that an ion sensor having highly stable sensorcharacteristics for no less than about one month can be fabricated byusing DOP as the plasticizer in the PVC ion carrier membrane of themembrane-coated electrode.

(2) It has been found that a large quantity of the plasticizer whichdissolves in the PVC influences the durability of the electrode (acoated wire-type electrode and ISFET electrode).

Further, according to the invention, there can be provided an ioncarrier membrane, and an ion sensor having the same, in which the ioncarrier membrane is prevented from dissolving and eluting in anelectrolytic polymeric layer (redox layer) and is improved in stabilitywith respect to the electrolytic polymeric layer by virtue of using aplasticizer selected from among polycaprolactone plasticizers andplasticizers comprising modified ethylene--vinyl acetate copolymer,which plasticizers do not readily dissolve in a solution undermeasurement, plasticize polyvinyl chloride resin in the ion carriermembrane and are compatible with the ion carrier material.

The following results are obtained:

(1) It has been found that a pH sensor having highly stable sensorcharacteristics for no less than about three months can be fabricated byselecting the plasticizer, which is used as the plasticizer in the PVCion carrier membrane of the membrane-coated electrode, from amongpolycaprolactone plasticizers and plasticizers comprising modifiedethylene--vinyl acetate copolymer. (In an acceleration test using anoven, it was found that durability could be increased more thanthree-fold over that obtained with the monomeric ester DOS usedconventionally.)

(2) In tests for elutants and elutant toxicity, it was found that theamount of elution of the polycaprolactone plasticizers and plasticizerscomprising modified ethylene--vinyl acetate copolymer was extremelysmall, and that the toxicity of the elutants posed no problem. Thismakes it possible to provide an ion sensor which can be used in bloodand in both in vivo and ex vivo measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural view of membrane-coated electrodes fabricated inExamples 1 through 5;

FIG. 2 is a view illustrating the membrane-coated electrode of FIG. 1undergoing measurement;

FIGS. 3(a) and 3(b) are views showing the characteristics of themembrane-coated electrodes of Examples 1 through 5;

FIGS. 4 and 5 are structural views of an ion sensor fabricated inExample 6;

FIGS. 6(a) and 6(b) are views showing the characteristics of amembrane-coated electrode in Example 7;

FIG. 7 is a structural view of membrane-coated electrodes fabricated inExamples 8 and 13;

FIG. 8(a) is a view showing temporal change in the sensitivity of a pHsensor fabricated in Examples 8 and 9; and

FIG. 8(b) is a view showing temporal change in the E^(o) of a pH sensorfabricated in Examples 8 and 9.

BEST MODE FOR CARRYING OUT THE INVENTION <Example 1>

The coated electrode shown in FIG. 1 was fabricated through thefollowing procedure: A columnar member 1 having a diameter of 1.56 mmwas cut from a sheet of basal plane pyrolytic graphite (BPG)(manufactured by UCC). A silver wire serving as a lead wire 7 wasconnected to one end of the BPG column 1 using an electricallyconductive adhesive 2 (C-850-6, manufactured by Amicon K.K.). Theexterior of the lead wire 7 was insulated with a heat-shrinkable tube 3,and the space between the tube 3 and the BPG 1 was insulated using aurethane adhesive 4. A redox layer 5 was formed on the surface of theBPG substrate by carrying out an electrolytic reaction, under theelectrolytic conditions shown below, in a three-pole cell using the BPGsubstrate, fabricated as set forth above, as a working electrode, areference electrode (a saturated sodium chloride calomel electrode,referred to as an "SSCE"), and a counter electrode (a platinum mesh).

Electrolyte: An acetonitrile solution of 0.2 M NaClO₄ and 0.5 M 2,6xylenol

Electrolytic reaction conditions: The electrolyzing voltage was sweptthree times (sweep rate: 50 mV/sec) from 0 V to 1.5 V (vs. the SSCE),followed by carrying out constant-potential electrolysis for 10 min at1.5 V (vs. the SSCE).

The redox layer so prepared was dark-red in color. After being washedand dried, the member fabricated above was dipped in a solution of ahydrogen ion carrier solution to form an ion carrier membrane 6 on theredox layer 5.

    ______________________________________                                        Composition of hydrogen ion carrier solution:                                 ______________________________________                                        tridodecyl amine        15.65 mg/ml                                           tetrakis(p-chlorophenyl) potassium borate                                                             1.565 mg/ml                                           di(2-ethylhexyl) phthalate (DOP)                                                                      162.8 mg/ml                                           polyvinyl chloride (p.sub.n -1050)                                                                    81.25 mg/ml                                           tetrahydrofuran (THF) (solvent)                                                                       10 ml                                                 Dipping conditions:                                                           dipping rate:           10 cm/min                                             number of times:        15 times                                              carrier membrane thickness:                                                                           about 0.7 mm                                          ______________________________________                                    

<Experiment No.1>

The circuit shown in FIG. 2 was used to measure a change in the Nernstresponse of an electrode obtained by coating the BPG substratefabricated in Example 1 with the electrolytic redox layer 5 and hydrogenion carrier membrane 6. The results are as shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                  Example No. 1  Example No. 2                                        Time      Eo:Slope (mV/pH)                                                                             Eo:Slope (mV/pH)                                     (Days)    of Nernst Equation                                                                           of Nernst Equation                                   ______________________________________                                        Initial   4 days required for                                                                          Same as at left                                      Stage     membrane stabilization                                               4 Days   502.83:63.04   508.59:62.50                                          7 Days   501.26:62.63   501.27:62.64                                         18 Days   504.22:62.89   492.38:60.84                                         25 Days   499.56:62.56   485.89:60.72                                         ______________________________________                                    

Comparison electrode: saturated sodium chloride calomel electrode (SSCE)

Temperature at measurement: 37° C.

<Example 2>

A membrane-coated electrode was fabricated as in Example 1 except forthe fact that dioctyl sebacate (DOS) was used instead of DOP as theplasticizer for forming the hydrogen ion carrier membrane 6 of Example1.

<Experiment No. 2>

The results of measuring pH characteristics using the membrane-coatedelectrode of Example 1 are as shown in Table 1. About seven days wererequired for the reference electrode potential (E_(o)) to stabilize.

<Examples 3-5>

A membrane-coated electrode was fabricated as in Example 1 except forthe fact that the types of plasticizer and solvent were changed, asshown in Table 2:

                  TABLE 2                                                         ______________________________________                                        Example   Plasticizer       Solvent                                           ______________________________________                                        3         dioctyl adipate (DOA)                                                                           THF                                               4         dioctyl maleate (DOM)                                                                           THF                                               5         dioctyl sebacate (DOS)                                                                          cyclohexane                                       ______________________________________                                    

<Experiment Nos. 3-5>

The results of measuring the durability of sensor characteristics(namely the reference electrode potential E_(o) and slope of the Nernstequation) using the membrane-coated electrodes of Examples 3-5 are asshown in Table 3. It should be noted that results similar to those shownfor Example 3 were obtained even with a carboxylic plasticizer[benzophenontetracarboxylic undecylic acid (BTCU)].

                  TABLE 3                                                         ______________________________________                                        Elapsed                                                                       Time    Example  3           4     5                                          ______________________________________                                         4 Days Eo       510.36      484.45                                                                              512.36                                             Slope    62.52       59.55 62.48                                       7 Days Eo       516.2       487.6 441.2                                              Slope    62.57       60.57 56.59                                      18 Days Eo       484.7       491.4 350.6                                              Slope    59.64       61.43 45.99                                      25 Days Eo       477.2       487.1 740.2                                              Slope    59.39       61.22 49.15                                      ______________________________________                                    

These results show that the reference electrode potential E_(o) andslope of the Nernst equation exhibit the best stability when the DOM-THFcombination is used.

It is evident that the DOA-THF combination results in a largefluctuation in E_(o), and that the DOS-cyclohexane combination causes alarge fluctuation in both E_(o) and the slope of the Nernst equation.

The overall results are illustrated in FIGS. 3(a) and 3(b).

<EXAMPLE 6>

An electrically conductive carbon membrane 15 was formed on the surfaceof a gate insulation layer 14 of a MOSFET, and a redox layer 16 andhydrogen ion carrier membrane 17 were formed on the carbon membrane 15as in Examples 1 and 7. The resulting ion sensor is illustrated in FIGS.4 and 5. The formation of the electrically conductive carbon membrane 15is described below.

(1) MOSFET

The MOSFET used was a FET (a so-called insulated-gate FET) having astructure in which p-type Si--SiO₂ gate insulation layers are formed ona p-type silicon wafer. The FET was fabricated on a p-type silicon waferutilizing an ordinary planar technique that relies uponphotolithography, and the result was coated with the insulating layer14, consisting of silicon nitride, using a sputtering process. Numeral11 denotes a drain, 12 a source, 13 an oxide film, and 18 a siliconsubstrate.

(2) Electrically conductive carbon membrane

An electrically conductive carbon membrane (membrane thickness: 2000 A)was formed as the electrically conductive layer 15 by an ion beamsputtering process on the surface of the gate insulation layer 14 of theMOSFET fabricated as described above.

<Experiment No. 6>

The electrode of Example 6 was tested as in Experiment No. 1 to measureits conductivity characteristics (the reference electrode potentialE_(o) and slope of the Nernst equation) and its durability. It wasclarified that results similar to those seen in Example 1 are obtained,and that the durability of the electrode is improved by using DOP as thehydrogen ion carrier membrane.

<Example 7>

A carbon electrode was fabricated as in Example 1, and the redox layer 3was deposited by electrolytic polymerization, as in Example 1. Thoughthe hydrogen ion carrier membrane 4 was fabricated using a method thesame as that employed in Example 1, the composition was changed to thatshown in Table 4.

                  TABLE 4                                                         ______________________________________                                        (Hydrogen Ion Carrier Composition                                             Adipate polyester (PN-250, manufactured by Adeka                              Argus K.K.) was used instead of dioctyl sebacate as the                       plasticizer.                                                                  ______________________________________                                        tridodecyl amine  40 mg      0.12 parts                                       tetrakis(p-chlorophenyl)                                                                        6 mg      0.018 parts                                       potassium borate                                                              polvinyl chloride                                                                              325 mg       100 parts                                       PN-250           650 mg       200 parts                                       tetrahydrofuran solvent                                                                         4 ml                                                        ______________________________________                                    

<Experiment No. 7>

Electromotive force with respect to a comparison electrode (SSCE) wasmeasured in a phosphate buffer solution using the pH sensor fabricatedin accordance with Example 7 as the active electrode, the electromotiveforce was plotted against pH, and the slope and reference electrodepotential E^(o) were measured with the passage of time. For comparisonpurposes, the electrode fabricated in Example 2 was also subjected tomeasurement in the same way. The results are as shown in Table 5 andFIGS. 6(a), 6(b).

These results show that the slope of 61.8+0.5 mV/pH (37° C.) up to 30days after fabrication closely approximates the theoretical value [61.5mV/pH (37° C.)] for all electrodes. This indicates good stability. Thereference electrode potential is also stable up to 30 days afterfabrication.

                  TABLE 5                                                         ______________________________________                                        Days After Fabrication                                                                      3 Days  10 Days  30 Days                                                                              90 Days                                 ______________________________________                                        PN-250 Electrode                                                                         Slope  -62.1   -61.7  -61.4  -60.9                                            Eo      478.1   480.2  479.5  469.8                                DOS Electrode                                                                            Slope  -62.5   -61.4  -61.7  -60.4                                            Eo      514.5   515.1  510.9  445.2                                ______________________________________                                    

<Experiment No. 8>

A severity test (acceleration test) was performed using the pH sensorfabricated in accordance with Example 7. In the test, a measurementsimilar to that described in Experiment No. 7 was carried out in an ovenat 60° C. while changing the electrode preservation conditions. Theresults are as shown in Table 6.

                  TABLE 6                                                         ______________________________________                                        Days After Introduction to Oven                                                                   1 Day   3 Days  10 Days                                   ______________________________________                                        PN-250 Electrode                                                                             Slope    -62.1   -61.7 -61.4                                                  Eo        478.1   480.2                                                                               479.5                                  DOS Electrode  Slope    -61.5   -61.4 -64.8                                                  Eo        514.5   515.1                                                                               442.1                                  ______________________________________                                    

<Experiment No. 9>

The influence of oxygen gas was investigated using the pH sensorfabricated in accordance with Example 7.

A mixed gas of oxygen and nitrogen having a constant concentration wasbubbled through a phosphate buffer solution having a pH of 7.4, and thepotential of the pH sensor was measured at various partial pressures,e.g. 200 mmHg, 150 mmHg, 400 mmHg, of the oxygen gas in the solution. Itwas found that the variation in potential was within 2 mV for allelectrodes, indicating no influence from oxygen gas.

The construction of a membrane-coated electrode in the followingExamples is illustrated in the sectional view of FIG. 7.

<Example 8> (1) Fabrication of electrode

A columnar member of BPG having a diameter of 1.0 mm and a length of 3.0mm and serving as an electrically conductive substrate 21 was cut from asheet of basal plane pyrolytic graphite (BPG) (manufactured by UnionCarbide Company). One end of the BPG 21 was cut down to a diameter of0.5 mm and a length of 1.5 mm, and a lead wire 25 (a polyethylenefluoride-coated copper wire) was bonded to this end of the substrateusing an electrically conductive adhesive 22 (C-850-6, manufactured byAmicon K.K.). The other end of the columnar substrate 21 having a lengthof 1.50 mm was ground and polished into a hemispherical shape, with thesurface area of the exposed portion being adjusted to an average valueof 0.064 cm². The end of the substrate 21 having the lead wire 25 wasinserted together with the lead wire 25 into a polyethylene fluoridetube 26 having an inner diameter of 0.60 mm and an outer diameter of0.85 mm, and this end of the substrate was bonded along with the leadwire 25 within the tube using an epoxy resin adhesive (TB2067,manufactured by Three Bond Corp.), thereby insulating the lead wire. Thetube 26 was then sheathed by a heat-shrinkable tube 27 to fabricate aBPG electrode.

Though such metals as platinum, gold, silver, copper, nickel and chromeand such compounds as indium oxide and iridium oxide can be used as theelectrically conductive substrate in the abovementioned electrode, BPGis particularly preferred since it is not affected by oxygen insolution.

(2) Application of redox layer 23 by electrolytic polymerization

An electrolytic oxidative polymerization reaction was carried out underthe following conditions using the above-described BPG electrode as aworking electrode, a reference electrode (a saturated sodium chloridecalomel electrode, or SSCE), and a counter electrode (a platinum mesh):

    ______________________________________                                        Electrolyte:    0.5M 2,6-dimethyl phenol                                                      0.2M sodium perchlorate                                                       acetonitrile solvent                                          ______________________________________                                    

Temperature at electrolysis: -20.0° C.

Electrolytic conditions: After the electrode potential was swept-threetimes from 0.0 V to 1.5 V (vs. the SSCE) at a sweep rate of 50 mV/sec),an electrolytic reaction was allowed occur at 1.5 V (vs. the SSCE) for10 min.

As the result of this electrolytic polymerization process, anelectrolytic oxidative polymer membrane of poly(2,6-xylenol) wasdeposited as the redox layer 23 to a thickness of about 30 microns onthe exposed surface of the BPG substrate 21.

A membrane thickness of 0.1 um-1 mm is preferred, especially thicknessof 20-100 um. A membrane thickness which is too great results in reducedspeed of response and sensitivity, while a membrane which is too thin isreadily influenced by the underlying electrically conductive substrate21.

(3) Application of hydrogen ion carrier membrane 24

The membrane-coated electrode obtained as set forth above was dipped ina hydrogen ion carrier composition shown in Table 7 as wall allowed todry. This operation was performed repeatedly 15 times, as a result ofwhich a hydrogen ion carrier membrane 24 having a thickness of 1.00 mmwas formed uniformly on the surface of the redox layer 23.

A membrane thickness of 0.1 um-2.0 mm is preferred, especially athickness of 0.5-1.5 mm. A membrane thickness which is too great resultsin an enlarged electrode diameter, while a membrane which is too thin ispoor in reproducibility.

The preferred amount of plasticizer is 100-300 parts by weight per 100parts by weight of polyvinyl chloride, with 150-250 parts by weightbeing particularly preferred. Too much plasticizer will result in atleast diminished sensitivity and, in particularly, reduced speed ofresponse with regard to changes in temperature.

                  TABLE 7                                                         ______________________________________                                        (Hydrogen Ion Carrier Composition)                                            ______________________________________                                        tridodecyl amine    40 mg     0.12 parts                                      tetrakis(p-chlorophenyl)                                                                          6 mg     0.018 parts                                      potassium borate                                                              polvinyl chloride  325 mg      100 parts                                      polycaprolactone plasticizer                                                                     650 mg      200 parts                                      tetrahydrofuran solvent                                                                           4 ml                                                      ______________________________________                                    

A polycaprolactone plasticizer was used as the plasticizer.

<Example 9>

(1) An electrode was fabricated as in Example 8, and (2) the redox layerwas deposited by electrolytic polymerization, as in Example 8.

(3) Application of hydrogen ion carrier membrane

Though a hydrogen ion carrier membrane was fabricated using a method thesame as that employed in Example 8, the composition was changed to thatshown in Table 4.

                  TABLE 8                                                         ______________________________________                                        (Hydrogen Ion Carrier Composition)                                            ______________________________________                                        tridodecyl amine    40 mg   0.12 parts                                        tetrakis(p-chlorophenyl)                                                                          6 mg   0.018 parts                                        potassium borate                                                              polvinyl chloride  325 mg    100 parts                                        Ervaroi 742        650 mg    200 parts                                        tetrahydrofuran solvent                                                                           4 ml                                                      ______________________________________                                    

Modified EVA (Ervaroi 742, manufactured by Mitsui Polychemicals K.K.)was used instead of polycaprolactone as the plasticizer.

<Example 10> (1) Fabrication of electrode

Instead of BPG, a graphite carbon material EG-51 (manufactured by NipponCarbon K.K.) was used as the electrically conductive substrate tofabricate an EG-51 electrode through the same procedure employed inExample 8.

(2) The application of the redox layer 23 by electrolytic polymerizationwas performed as in Example 8.

(3) The application of the hydrogen ion carrier membrane 24 wasperformed as in Example 8 using the polycaprolactone plasticizer.

<Example 11> (1) Fabrication of electrode

Instead of BPG, a graphite carbon material T-5 (manufactured by IBIDENK.K.) was used as the electrically conductive substrate to fabricate aT-5 electrode through the same procedure employed in Example 8.

(2) The application of the redox layer 23 by electrolytic polymerizationwas performed as in Example 8.

(3) The application of the hydrogen ion carrier membrane 24 wasperformed as in Example 10.

<Example 12> (1) Fabrication of electrode

Instead of BPG, a carbon material, namely the core of an automaticpencil, was used as the electrically conductive substrate to fabricatepencil-core electrode through the same procedure employed in Example 8.

(2) The application of the redox layer 23 by electrolytic polymerizationwas performed as in Example 8.

(3) The application of the hydrogen ion carrier membrane 24 wasperformed as in Example 11.

<Experiment No. 10>

Electromotive force with respect to a comparison electrode (SSCE) wasmeasured in a phosphate buffer solution using the pH sensor fabricatedin accordance with Example 9 as the working electrode, the electromotiveforce was plotted against pH, and the slope and reference electrodepotential E_(o) were measured with the passage of time. For comparisonpurposes, the electrode fabricated in Example 1 was also subjected tomeasurement in the same way. The results are as shown in Table 9 andFIGS. 8(a), 8(b).

                  TABLE 9                                                         ______________________________________                                        Days After Fabrication                                                                      3 Days  10 Days  30 Days                                                                              90 Days                                 ______________________________________                                        Polymeric (Poly-                                                                         Slope  -61.3   -61.6  -61.4  -60.9                                 caprolactone)                                                                            Eo      501.4   503.2  505.9  495.8                                Plasticizer                                                                   Electrode                                                                     Ervaroi 742                                                                              Slope  -62.5   -62.5  -62.7  -62.3                                 Electrode  Eo      507.2   508.9  507.5  502.5                                ______________________________________                                    

These results show that the slope of 61.8+0.5 mV/pH (37° C.) up to 30days after fabrication closely approximates the theoretical value [61.5mV/pH (37° C.)] for all electrodes. This indicates good stability. Thereference electrode potential was also stable up to 30 days afterfabrication. More than thirty days after fabrication, however, thereference electrode potential began to drop. After 90 days, thereference electrode potential dropped by 60 mV in comparison with thestable period. The electrode of the present example using the polymericplasticizer in order to prevent the outflow of the redox layer into theplasticizer exhibited a change in reference electrode potential within+10 mV, even after 90 days. This fact demonstrates that the outflow ofthe redox layer into the plasticizer is prevented, thereby extendingdurability.

<Experiment No. 11>

A severity test (acceleration test) was performed using the pH sensorfabricated in accordance with Example 9. In the test, a measurementsimilar to that described in Experiment No. 10 was carried out in anoven at 60° C. while changing the electrode preservation conditions. Theresults are as shown in Table 10.

                  TABLE 10                                                        ______________________________________                                        Days After Introduction to Oven                                                                  1 Day   3 Days  10 Days                                    ______________________________________                                        polycaprolactone                                                                            Slope    -62.3   -61.6 -61.4                                    Plasticizer Electrode                                                                       Eo        501.4   503.2                                                                               505.9                                   Ervaroi 742 Electrode                                                                       Slope    -62.1   -62.4 -61.9                                                  Eo        507.7   509.2                                                                               507.1                                   ______________________________________                                    

From the fact that the slope and reference electrode potentialassociated with the electrode using the polymeric plasticizer were notfound to exhibit changes in excess of ±0.5 mV/pH and ±10 mV,respectively, ten days after introduction to the oven, it was confirmedthat elution of the redox layer was prevented.

<Experiment No. 12>

Upon conducting an experiment the same as that of Experiment No. 10using pH sensors fabricated in accordance with Examples 10, 11 and 12,it was found that the pH sensors responded linearly in closeapproximation to the theoretical slope, with respect to a variation inpH, even when the carbon substrate was changed. In addition, when thepolycaprolactone polymeric plasticizer was used as the plasticizer, theelution of the redox layer could be prevented and durability wasimproved. Thus, results similar to those seen in Experiment No. 10 wereobtained.

<Experiment No. 13>

The influence of oxygen gas was investigated using the pH sensorsfabricated in accordance with Examples 8, 9, 10, 11 and 12.

A mixed gas of oxygen and nitrogen having a constant concentration wasbubbled through a phosphate buffer solution having a pH of 7.4, and thepotential of the pH sensor was measured at various partial pressures,e.g. 200 mmHg, 150 mmHg, 400 mmHg, of the oxygen gas in the solution. Itwas found that the variation in potential was within 2 mV for allelectrodes, indicating no influence from oxygen gas.

<Example 13>

An electrically conductive carbon membrane 15 was formed on the surfaceof a gate insulation layer 14 of a MOSFET, and a redox layer 16 andhydrogen ion carrier membrane 17 were formed on the carbon membrane 15as in Example 8. The structure of the resulting ion sensor was the sameas that shown in FIGS. 4 and 5. The formation of the electricallyconductive carbon membrane 15 is described below.

(1) MOSFET

The MOSFET used was a FET (a so-called insulated-gate FET) having astructure in which p-type Si--SiO₂ gate insulation layers are formed ona p-type silicon wafer. The FET was fabricated on a p-type silicon waferutilizing an ordinary planar technique that relies uponphotolithography, and the result was coated with the insulating layer14, consisting of silicon nitride, using a CVD (chemical vapordeposition) process. Numeral 11 denotes a drain, 12 a source, 13 anoxide film, and 18 a silicon substrate.

(2) Electrically conductive carbon membrane

The electrically conductive carbon membrane 15 was formed by an ion beamsputtering process to a thickness of 2000 A on the surface of the gateinsulation layer 14 of the MOSFET fabricated as described above.

<Experiment No. 14>

The electrode of Example 13 was tested as in Experiment No. 10 tomeasure its conductivity characteristics (the reference electrodepotential E_(o) and slope of the Nernst equation) and its durability. Itwas clarified that results similar to those seen in Example 8 areobtained, and that the durability of the electrode is improved by usinga polycaprolactone polymer as the plasticizer of the hydrogen ioncarrier membrane.

Thus, as will be understood from the foregoing Examples, the pHelectrodes of these Examples using a polymeric plasticizer prevent theelution of the redox layer, exhibit excellent stability and durabilityand are not affected by oxygen gas. In addition, the 95% speed ofresponse was rapid, being within four seconds.

The polymeric plasticizers used herein have the following structures:##STR1## where P is the modified portion ##STR2##

Though the foregoing examples have been described using a hydrogen ioncarrier membrane as a typical ion carrier membrane, similar effects areobtained with other ion carrier membranes as well. Furthermore, theelectrically conductive substrates and electrically conductive layersare not limited to carbon; metals such as gold, platinum and nickel canalso be used. As for the carbon, that having a graphite-type crystalstructure is preferred for use. In addition, the structures of themembrane-coated electrodes and FET sensors are not limited to thosedescribed in the Examples.

We claim:
 1. An ion carrier membrane sensitive to a prescribed ion andcomprising polyvinyl chloride plasticized by a plasticizer,saidplasticizer being benzophenonetetracarboxylic undecylic acid (BTCU). 2.An ion sensor comprising an electrically conductive substrate, a redoxlayer exhibiting a redox function coating said electrically conductivesubstrate, and an ion carrier membrane coating said redox layer,whereinsaid ion carrier membrane is sensitive to a prescribed ion and comprisespolyvinyl chloride plasticized by a plasticizer, said plasticizer beingbenzophenontetracarboxylic undecylic acid (BTCU).
 3. An ion sensorcomprising a gate insulation layer of a FET, an electrically conductivelayer coating said gate insulation layer, a redox layer exhibiting aredox function coating said electrically conductive layer, and an ioncarrier membrane coating redox layer,wherein said ion carrier membraneis sensitive to a prescribed ion and comprises polyvinyl chlorideplasticized by a plasticizer, said plasticizer beingbenzophenontetracarboxylic undecylic acid (BTCU).